Due to the wide variety of subject areas this invention encompasses, there is a tremendous amount of background information that can be examined. The following sections will identify areas that are helpful toward the understanding of the invention.
Neutron Generation
There are many available means to generate neutrons: such generators include nuclear reactors, radioisotope sources, linear accelerator spallation devices, accelerator-solid target devices, and a host of plasma-confinement fusion concepts.
Nuclear reactors can produce tremendous quantities of neutrons, but are only considered for very large-scale applications that would require a dedicated facility. Reactors are often quite large, expensive to construct and operate, and are heavily regulated. They are not considered portable or cost-effective for small-scale neutron applications.
Radioisotope neutron sources consist of an isotope that spontaneously fissions, such as californium-252, or an alpha-emitter, such as plutonium-240 or americium-241, mixed with beryllium. Radioisotope sources are often small and can provide large neutron fluxes, however, they cannot be “turned off” and must be shielded to protect personnel from radiation exposure. Such sources produce neutrons at various energies, as well as other types of radiation, including beta, gamma, and x-rays. The health risks, costs and ultimate disposal issues of such neutron sources make them unattractive for industrial applications.
Linear accelerator spallation sources accelerate ions to high energies and send them into a target to produce neutrons from spallation reactions. Neutrons produced from the spallation reactions are emitted in a “forward-peaked” direction, resulting in an intense, directed neutron flux, which can be useful in some applications. The spallation targets can produce large neutron yields and can have a reasonably long lifetime. However, these devices require large support structures, are not considered portable, and are very expensive to construct and operate.
There are many concepts that use the fusion process to generate neutrons. Fusion is a nuclear process where two light nuclei undergo a high-energy collision that rearranges the subatomic particles of the colliding nuclei to form two (or more) different nuclei with high energies. There are two principal fusion reactions that produce neutrons. One is the reaction between deuterium and tritium, which produces 14.1 MeV neutrons, along with 3.5 MeV alpha particles. The other key neutron producing reaction is between two deuterium nuclei; however, this reaction has two possible outcomes: a 2.45-MeV neutron and a 0.82-MeV helium-3 nucleus, or a 3.06-MeV proton and a 1.02-MeV triton (tritium nucleus).
Accelerator-solid target devices utilize the acceleration of deuterons, usually generated with a Penning ion source, into a target loaded with deuterium or tritium in solid solution or hydride form. The energies of acceleration are in the hundreds of kilovolts to allow enough penetration of the deuteron into the target metal lattice to find a D or T atom for fusion. These devices operate with a low background gas pressure and a high target density within the metal lattice to maximize fusion probability and neutron output. However, the targets erode from particle impingement and thermal degradation, resulting in minimal lifetimes measured in hundreds of hours with nominal neutron output. These sources are normally pulsed to generate high neutron fluxes and to reduce target degradation.
There are several plasma-confinement fusion concepts capable of generating neutrons that have been developed in the hope of commercially generating electrical power. There are two “mainstream” categories of fusion devices, magnetically confined fusion (MCF), and inertial confinement fusion (ICF). In MCF systems such as tokamaks, spheromaks and stellerators, strong magnetic fields are used to confine a plasma with a temperature sufficient for fusion, thereby generating neutrons. In ICF systems, a small pellet or droplet of fusible material is rapidly heated and compressed by high-energy laser beams or particle beams to cause fusion, thereby generating neutrons. Generally, the “mainstream” MCF and ICF systems are very large and require large amounts of power and support infrastructure to generate large quantities of neutrons; thus these systems are not practical for industrial neutron analysis.
There are also many lesser-known “alternative” plasma-confinement fusion concepts that are more practical for neutron production, due to their smaller size. Theta-pinch and z-pinch devices attempt to confine high-energy plasma with self-generated magnetic fields caused by flowing current. Plasma focus devices use a transient discharge current to push fusible gas down the length of an electrode, heating and compressing it at the end of the electrode. These systems provide moderate bursts of neutrons but with low repetition rates. Inertial electrostatic confinement systems establish “virtual” electrode confinement regions, called “poissors”, that confine energetic ions at a high-density where they can fuse with each other, however such systems require large volumes and large support structures for practical neutron generation.
Accelerator Gas-Target Neutron Generation
A new method of neutron generation employs an accelerator to send high-energy ions into a gas target for the fusion reaction. The idea of using high-energy ions to interact with a gas is not new, but applying the idea to create a portable neutron generator is new. Accelerator gas-target devices generally contain an ion source region, acceleration region and gas-target region. There are potential advantages to the gas-target system due to the constant replenishment of target material and the removal of the degradable metal lattice found in solid-target systems. However, there are disadvantages in power efficiency due to electron generation and mobility, and low gas-target density for the fusion reactions necessary for efficient neutron generation. Another aspect of the gas-target neutron source is that the gas-target region can be of varying geometry to produce different shaped neutron production regions.
Electron Management System
Another background field for the invention contained herein is in the realm of electron management. Gas-target systems do not benefit from a metal lattice to absorb electrons generated from the high-energy ion impact with the target material. Consequently, electrons are generally free to be accelerated through the acceleration region to the detriment of the gas-target system, limiting performance. Thus, electron management can be applied to the gas-target system to increase system efficiency, including techniques such as suppression, repression, screening, extraction, and collection. The adoption of electron management principles and methods allow gas-target systems to provide equivalent or superior performance to that of conventional solid-target accelerators.
Gaseous Discharges
As stated, the invention described in this document utilizes a gaseous discharge process that operates in a high-pressure high-resistance mode to generate high-energy particles for the preferred embodiment of the family of gas-target systems. The HPHRGD neutron generator operates in a weakly ionized state where collisions between heavy particles provide the ionizations to sustain the discharge. Operation in this state is commercially unattractive for processing applications, due to low plasma generation densities per unit input power. However, the high-pressure high-resistance gaseous discharge does produce many high-energy particles that are applicable to nuclear interactions and other collisional processes.
Background information in the field of plasma discharge devices is broad and varied. The most common and prevalent plasma devices are radio frequency driven devices and direct current glow discharges. These discharge devices generally operate in the low-resistance electron-dominated “normal glow” discharge mode at low voltages (hundreds of volts). Some glow discharge systems can operate with larger potential differences, but operate in the “abnormal glow” or “obstructed glow” regimes of electron dominance over plasma sheath distances.
The high-pressure high-resistance gaseous discharge described in this document is not a “glow discharge” as defined in conventional plasma physics. Rather, it could be considered as a hybrid between a particle accelerator and an electrical discharge. The HPHRGD neutron generator operates with a gaseous discharge that is sustained primarily by ionizing collisions between heavy-particles (ions and fast neutral gas particles) over long path lengths, which has a higher resistance than other electron-dominated discharges. The HPHRGD system shares some aspects of conventional plasma devices, such as pseudosparks, thyratrons, vacuum switches, and aspects of high-energy accelerators as well. The phrase “high-pressure” is associated with the successive electron management improvements that allow higher gas-target pressures for increased neutron generation.
Electrode geometry is an important aspect of the innovation since it is a defining factor in the formation of the high-pressure high-resistance gaseous discharge. The use of a semi-transparent cathode (an electrode with openings or holes) in the HPHRGD system is to provide transparency for ions and fast-neutral particles to transit to other regions of the device. The semi-transparent electrode is not meant to trap or confine any particles within the discharge, as is common with “hollow-cathode” systems in the traditional sense, which effectively confine electrons through a reflexing action long enough to provide large electron-generated plasma densities. The hollow cathode effect is useful in enhancing electron-dominated discharges and increasing the efficiency of plasma generation. The electrodes of the HPHRGD system are designed to prevent the traditional hollow cathode effect from occurring during normal operation.
HPHRGD Neutron Generator Components
The HPHRGD embodiment of the gas-target neutron generator combines the ion source, acceleration region, and gas-target volume components of a basic gas-target system into one simple device. The invention utilizes various gaseous discharge geometries to create different neutron source distributions, such as linear, planar, and annular. The choice of neutron source geometry can improve analysis techniques by providing neutrons where they are needed to perform effective scanning in various applications. Also, these geometries allow for efficient cooling of the discharge device during high-power operation. As a result, the HPHRGD neutron generator can operate with greater average power inputs than other neutron sources, such as accelerator-solid-target sources.
The HPHRGD neutron generator uses semi-transparent electrodes to allow particles to traverse the neutron generator. The electrode openings also allow electric fields to penetrate inside the electrode. The size and number of electrode openings can be adapted to decrease the potential differences within the cathode region, thereby repressing secondary electron emission. The concept of altering electrode openings to reduce electric field penetration is fairly common, such as using this technique to repress secondary electron emission in vacuum tubes and thyratron switches. However, the application of this technique to increase the gaseous discharge resistance to improve neutron generation is new.
Another improvement to the HPHRGD neutron generator suppresses and removes unwanted electrons from the device for increased neutron production power efficiency. Electron suppression techniques have been employed in detection systems and over-current devices to focus, reflect, inhibit, or minimize particle flow in one direction for improved efficiency. Removing unwanted charged particles in specific regions of the high-pressure high-resistance gaseous discharge can improve the system performance by altering the pressure-distance parameter and constituents of electrical current passing through the gas. Passive (un-powered) charged particle removal techniques employing recombination can also increase the performance of the HPHRGD neutron generator. Applications that employ similar techniques include material electron absorbers in plasma switches, thyratrons, igniters, triodes, and high-intensity plasma lamps. The use of such techniques to increase discharge resistance to improve neutron generation is novel.
The performance of the HPHRGD neutron generator is further improved by utilizing materials that provide specific surface effects, such as increased or reduced secondary electron emission, increased particle reflection and gas absorption on surfaces. Electron emission near an electrode can lead to localized ionization of the background gas, which may be desired in certain areas and undesired in others. Increased gas absorption and particle reflection from electrode surfaces lead to increased ion ejection rates off of those surfaces. Various applications, such as lighting systems, discharge lamps, mutlipactors, and over current-arrestors, use material choice and surface treatments to achieve desired ionization effects and to extend electrode lifetime. The use of surface materials to increase gaseous discharge resistance and enhance neutron generation is a new application.
A comprehensive control system for the HPHRGD neutron generator is desired to regulate all of the system components and monitor parameter conditions. A single package of hardware and/or software is envisioned to integrate the control of the HPHRGD neutron generator with the control of radiation data acquisition systems, data analysis programs, and other industrial process systems. While there are several parameters that can be monitored and/or regulated, this task is within the computational capabilities of existing industrial controllers.
Neutron-Based Applications
There is a plethora of background information in the field of neutron assay and interrogation. Prompt and delayed gamma neutron activation analysis and neutron thermalization analysis techniques date back to the late 1930s. Non-destructive evaluation (NDE) techniques were improved in the 1950s and 1960s when commercial neutron sources became more widespread. FIG. 3 presents generalizations about various neutron material analysis techniques. These analysis techniques can be implemented to utilize the unique properties of the gas-target HPHRGD neutron generator, specifically its neutron generation source geometry.
Prompt Gamma Neutron Analysis (PGNA) is a nuclear technique to determine the presence of chemical elements within a material. Neutrons strike the nuclei of a material and some neutrons are absorbed or deflected, transferring energy to the nuclei. As shown in FIG. 3a, isotopes will promptly emit gamma rays with a characteristic energy that can be used to identify the chemical element (specifically, the isotope). By measuring the energies and the quantities of the gamma rays immediately released, it is possible to determine the amounts of chemical elements within the material. Certain elements will produce prompt gammas with thermal neutrons and others with fast neutrons, each producing a characteristic gamma ray that can be detected. Such measurements can be made in real time, allowing for online industrial process control and rapid element identification and imaging for security or environmental monitoring purposes.
Delayed Gamma Neutron Analysis (DGNA) is quite similar to PGNA. As indicated in FIG. 3b, the key difference is that in DGNA, the nuclei that absorb neutrons become radioactive and emit characteristic gamma rays (or other radiation) over a period of time that corresponds to the activated isotope's decay constant. This time delay could also be used in conjunction with the gamma-ray energy to identify the isotopes that have been activated. Like PGNA, DGNA can be used with neutrons of almost any energy to help determine the chemical composition.
Neutron thermalization analysis is a technique where neutron energy distributions are measured before and after the neutrons pass through a material, as shown in FIGS. 3c, 3d, and 3e. By examining the change in energy as the neutrons pass through material, various material properties can be calculated, such as thermalization lengths, average atomic mass, density, thickness, porosity, hydrogen content, and moisture content. Such measurements can be performed in real time to provide material analysis for industrial process control. This technique requires a source of fast neutrons, such as the gas-target neutron generator, because neutron energy losses are measured to determine the desired material properties.
On-line industrial process control based on material analysis is employed in many applications. X-rays, gamma rays, and lasers are often used to measure thicknesses and densities to maintain quality product control. Infrared light is used to measure the content of moisture and other chemical compounds. Chemical composition can be continuously monitored to sort material based on its content. Material properties can be used to maintain quality control of a product as it is being formed. The described neutron assay techniques can be used independently or in concert to determine material properties needed for industrial process control. Using energy sensitive gamma detectors and coincidence counting methods, it is possible to use DGNA and PGNA to measure the elemental content of a material and its flow rate or velocity. These techniques can be applied effectively using an HPHRGD neutron generator, which provides the added benefits of better geometry, longer life, shorter computation time from increased source strength, safety and portability.
A more specific application of the material analysis techniques is the inspection of closed packages for security at airports and other shipping facilities. Such inspections are commonly made with x-ray imaging, metal detectors, chemical vapor detection equipment, and by dogs trained to find explosives or contraband. By irradiating baggage and cargo with neutrons, the elemental contents of the interrogated item can be determined. The characteristic gamma rays detected by energy-sensitive detectors can indicate the elements that are present, the total amount of each element, and the location of the elements within the closed package. Systems for neutron interrogation of baggage are commercially available, but have not gained widespread acceptance. The gas-target HPHRGD neutron generator can provide neutrons continuously, reliably, and safely.
Environmental analysis applications often employ chemical identification techniques, such as spectral analysis and gas chromatography, to monitor elemental content. Such techniques often require material to be sampled and analyzed at an off-site laboratory. Many environmental applications would benefit from a neutron source that can be operated “in the field” and moved to various locations to make a series of measurements. The in situ measurement of soil properties (such as nutrient content, moisture content, and trace contaminants) can be made with a neutron generator, energy-sensitive gamma detectors, and related equipment on a small trailer towed by a combine or tractor. The same equipment can be placed on an extension boom of a remote-controlled vehicle to safely locate landmines in a minefield. The gas-target HPHRGD neutron generator can generate neutrons over a wide area to provide rapid measurements, and is small and rugged enough to be placed on a cart or other mobile platform for such tasks.
Neutron sources can also be utilized in various medical applications. Medical imaging often involves the detection of x-rays, magnetic resonances, or gamma rays. Another imaging method involves the use of neutron-sensitive drug tracers that produce gamma rays only when bombarded by neutrons. The detection of such gamma rays can be used to determine their point of origin and produce an image of that tissue area. Various forms of radiation have been used in the treatment of diseases, including heat, lasers, x-rays, gamma-ray, and proton beams. Neutrons can also be used to treat cancer, typically in conjunction with elements that readily absorb neutrons to release other forms of radiation, such as alpha particles. Alpha particles can quickly deposit their energy within a biological cell, breaking chemical bonds 31 and destroying that cell's structure, as shown in FIG. 4b. By loading cancerous cells 30 with isotopes that absorb neutrons 28 to emit alphas, such as boron-10 29 (as shown in FIG. 4a), a neutron source can be used to destroy cancerous cells without adversely affecting the surrounding healthy tissue.
Neutron analysis techniques and applications benefit from using gas-target neutron generation. Gas-target sources have advantages over existing sources because they can be installed in existing facilities, and have a consistent output over a long lifetime, allowing for continuous on-line material analysis and process control. The HPHRGD system can be configured in almost any geometry to deliver neutrons to best suit the application or analysis technique. When the gas-target source is not in use, it can be turned off to stop neutron generation and stored safely with little to no radiation shielding.